The particular status of estuaries, deltas and lagoons in coastal areas, located at the interface between sea and river influences, results in highly variable environmental and ecological conditions that shift over space and time. The combined effects of climatic changes and human activities have tremendous consequences on these ecosystems. The conservation of these environments is one of the biggest challenges for humanity. In order to achieve an integrated management, researchers, ecologists and managers try to select relevant indicators, which could be used as tracers of the state of estuarine areas. These indicators are generally chosen among living species or physicochemical parameters or a combination of both. Among the fish species living in estuarine ecosystems, very few are able to live in a wide diversity of habitats. It is yet the case of one peculiar species, Mugil cephalus (Mugilidae), which is found worldwide and is cosmopolitan in almost all tropical coastal estuarine zones, but also in temperate ones. This species is able to live and reproduce in widely different habitats. The mechanisms, which are involved in this process, are poorly known or are studied separately in each area. Moreover this species and related ones support important fisheries, especially in developing countries. The aim of the MUGIL project is to build an observation network and to coordinate across the world the action of using this species as an indicator of the state of estuarine areas by observing the population genetics, the life history trait variations and the physiological responses to salinity or pollution. The project covers four world areas (Europe, Africa, Asia, America) and involves collaborators from southern Europe (France, Spain, Greece) and tropical countries (Mexico, Senegal, Benin, South Africa, Taiwan). Finally, it aims at proposing this species as a tool to follow littoral environmental changes through a new STREP proposal within the 7th Framework Programme.